1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device. In particular, the invention relates to a method for manufacturing a semiconductor device having an electrode structure that has an appropriate reflectance and does not generate a void.
2. Description of the Related Art
Hitherto, semiconductor devices using silicon (Si) as a semiconductor material have been mainly used as power devices. Silicon carbide (SiC) which is a wide gap semiconductor has the following physical property values: thermal conductivity that is three times more than that of silicon; maximum electric field intensity that is ten times more than that of silicon; and an electron drift speed that is two times more than that of silicon. Therefore, each institute has conducted research on an application technique using SiC since a power device using SiC has a high breakdown voltage and low loss and can operate at a high temperature.
As the structure of the power device, a vertical semiconductor device has been mainly used in which a rear surface electrode including a low-resistance ohmic electrode is provided on the rear surface side of a substrate. The rear surface electrode is made of various materials and has various structures. As an example of the structures, for example, a laminate of a titanium layer, a nickel layer, and a silver layer (for example, see JP 2007-184571 A, Patent Document 1) or a laminate of a titanium layer, a nickel layer, and a gold layer (for example, see JP 2010-86999 A, Patent Document 2) has been proposed.
In a vertical semiconductor device using SiC which is typified by a Schottky barrier diode, a method has been used which forms a nickel layer on a SiC substrate, heats the nickel layer to form a nickel silicide layer, and forms an ohmic contact between the SiC substrate and the nickel silicide layer (for example, see Patent Documents 1 and 2).
As a method for forming an ohmic electrode, a method has been proposed which forms a plurality of metal films on a SiC substrate and performs a heating process at a temperature of 700° C. to 1100° C., and most preferably, about 800° C. to obtain good ohmic characteristics (for example, see JP 2005-277240 A, Patent Document 3). Further, a technique has been proposed which radiates a laser beam to form an ohmic electrode (for example, see JP 2008-135611 A, Patent Document 4).
For a surface electrode which is formed on the front surface of the SiC substrate, a metal layer that is made of aluminum or aluminum alloy is formed on the Schottky contact to form the electrode structure. When the Schottky contact is not completely covered with the metal layer, a void is generated and causes an element defect.
In particular, the surface electrode of the SiC semiconductor needs to closely cover the uneven pattern of the Schottky contact (Schottky electrode). In addition, the surface electrode needs to be formed so as to have reflectance which is the most suitable for image recognition, such as positioning, when automatic wire bonding is performed.
The invention has been made in view of the above-mentioned problems and an object of the invention is to provide a method for manufacturing a semiconductor device which closely covers an uneven portion of a Schottky contact. In addition, an object of the invention is to provide a method for manufacturing a semiconductor device which can form a surface electrode having reflectance that is the most suitable for image recognition such as positioning.